In recent years, resistive changes in detector elements due to elongation and compression were detected by strain gauges, but there was no method for measuring fatigue. However, recently methods have been adopted wherein magnetic changes in structural materials were observed by use of detectors. In such cases, the measuring devices were large-scale and it was difficult to use them in actual job sites. As examples, 1988, J. Appl. Phys. 63(8) April 15, D. C. Jiles, Iowa State Univ. and J. Appl. Phys. 75(10) May 15, 1994, Z. J. Chen, D. C. Jiles, J. Kamedas' "Estimation of Fatigue Exposure from Magnetic Coercivity" cover this subject matter.
As summarized in J. Appl. Phys. 75(10) May 15, 1994 paper, magnetic structural steel materials were represented by H.infin. ln(N), expressing the relationship between stress cycles (N) and coercivity (H), indicating a fixed linearity existing between the stress cycles (N) and coercivity (H). However, this relationship applies only to structural steel materials and not to magnetic steel materials which are exceedingly corrosion prone. Furthermore, when a magnetic measuring device is attached to the material structure itself, it is the same method that is presently being used for measuring magnetic strain.
Generally speaking, magnetic substances are very hard, brittle and difficult to machine. Furthermore, they are very corrosive and unstable. Since they also become brittle under repeated stress cycling and rupture easily, it is vital that noncorrosive and stable materials be selected. Devices for magnetically detecting stress, strain, and fatigue in various materials must be capable of maintaining stability over at least ten years, and they must be small in size so as to permit easy attachment and detachment.